The solid angle computation for the sampling/PDF code was running into
cancellation issues for small solid angles.
Reformulating the terms using asin() instead avoids this.
based on concentric disk mapping.
Concentric disk mapping was already present, but not used everywhere.
Now `sample_cos_hemisphere()`, `sample_uniform_hemisphere()`, and
`sample_uniform_cone()` use concentric disk mapping.
This changes the noise in many test images.
Pull Request: https://projects.blender.org/blender/blender/pulls/109774
This reverts commit 206ab6437b.
Seems that the illegal address error should be covered elsewhere, but it's not directly
clear where. Revert the commit for further investigation.
Discovered during an investigation into #111277
in rare situations (E.G. When normals are NaN), an emitter
won't be selected as part of `light_tree_cluster_select_emitter()`
and as a result of that, an `emitter_index` of `-1` is passed to
`kernel_data_fetch(light_tree_emitters, emitter_index)` resulting in
an "illegal address" error on some devices.
Pull Request: https://projects.blender.org/blender/blender/pulls/111292
This pull request covers up a subtle difference between the CPU and GPU
when rendering with a light tree. Specifically a case where the user
has a sun light with a small angle.
The difference was caused by the dot() function being different between
CPU and GPU backends, with the GPU showing more meaningful
floating-point precision losses when working with small suns.
Pull Request: https://projects.blender.org/blender/blender/pulls/110307
The issue was an out-of-bounds read access when checking whether
the world volume emission needs to be accumulated.
Solution is to check for this case. Done in the generic place, so
that the shade_volume kernel is more readable and no branching
added there, and there is no impact on scenes without the light
linking.
Assume that the world emissive volume belongs to the default light
linking group, as there is no way to link it explicitly to anything.
Pull Request: https://projects.blender.org/blender/blender/pulls/110733
Using area-preserving mapping from cone to disk. Has somewhat distortion
near 90°.
The texture rotates with the transformation of the light object, can
have negative and non-uniform scaling.
Pull Request: https://projects.blender.org/blender/blender/pulls/109842
This fixes the issue described in https://projects.blender.org/blender/blender/issues/108957.
Instead of modeling distant lights like a disk light at infinity, it models them as cones. This way, the radiance is constant across the entire range of directions that it covers.
For smaller angles, the difference is very subtle, but for very large angles it becomes obvious (here's the file from #108957, the angle is 179°):
| Old | New |
| - | - |
|  |  |
One notable detail is the sampling method: Using `sample_uniform_cone` can increase noise, since the sampling method no longer preserves the stratification of the samples. This is visible in the "light tree multi distant" test scene.
Turns out we can do better, and after a bit of testing I found a way to adapt the concentric Shirley mapping to uniform cone sampling. I hope the comment explains the logic behind it reasonably well.
Here's the result, note that even the noise distribution is the same when using the new sampling:
| Method | Old | New, basic sampling | New, concentric sampling |
| - | - |- | - |
| Image |  |  |  |
| Render time (at higher spp)| 9.03sec | 8.79sec | 8.96sec |
I'm not sure if I got the `light->normalized` handling right, since I don't really know what the expectation from Hydra is here.
Co-authored-by: Weizhen Huang <weizhen@blender.org>
Pull Request: https://projects.blender.org/blender/blender/pulls/108996
The spotlight is now treated as a sphere instead of a view-aligned disk.
The implementation remains almost identical to that of a point light,
except for the spotlight attenuation and spot blend. There is no
attenuation inside the sphere. Ref #108505
Other changes include:
## Sampling
Instead of sampling the disk area, the new implementation samples either
the cone of the visible portion on the sphere or the spread cone, based
on which cone has a smaller solid angle. This reduces noise when the
spotlight has a large radius and a small spread angle.
| Before | After |
| -- | -- |
||
## Texture
Spot light can now project texture using UV coordinates.
<video src="/attachments/6db989d2-7a3c-4b41-9340-f5690d48c4fb"
title="spot_light_texture.mp4" controls></video>
## Normalization
Previously, the normalization factor for the spotlight was \(\pi r^2\),
the area of a disk. This factor has been adjusted to \(4\pi r^2\) to
account for the surface area of a sphere. This change also affects point
light since they share the same kernel type.
## Versioning
Some pipeline uses the `Normal` socket of the Texture Coordinate node for
projection, because `ls->Ng` was set to the incoming direction at the
current shading point. Now that `ls->Ng` corresponds to the normal
direction of a point on the sphere (except when the radius is zero),
we replace these nodes with a combination of the Geometry shader node
and the Vector Transform node, which gives the same result as before.
Example file see https://archive.blender.org/developer/T93676
Pull Request: https://projects.blender.org/blender/blender/pulls/109329
for energy preservation and better compatibility with other renderes. Ref: #108505
Point light now behaves the same as a spherical mesh light with the same overall energy (scaling from emission strength to power is \(4\pi^2R^2\)).
# Cycles
## Comparison
| Mesh Light | This patch | Previous behavior |
| -------- | -------- | -------- |
|  |  |  |
The behavior stays the same when `radius = 0`.
| This patch | Previous behavior |
| -------- | -------- |
|  |  |
No obvious performance change observed.
## Sampling
When shading point lies outside the sphere, sample the spanned solid angle uniformly.
When shading point lies inside the sphere, sample spherical direction uniformly when inside volume or the surface is transmissive, otherwise sample cosine-weighted upper hemisphere.
## Light Tree
When shading point lies outside the sphere, treat as a disk light spanning the same solid angle.
When shading point lies inside the sphere, it behaves like a background light, with estimated outgoing radiance
\[L_o=\int f_aL_i\cos\theta_i\mathrm{d}\omega_i=\int f_a\frac{E}{\pi r^2}\cos\theta_i\mathrm{d}\omega_i\approx f_a \frac{E}{r^2}\],
with \(f_a\) being the BSDF and \(E\) `measure.energy` in `light_tree.cpp`.
The importance calculation for `LIGHT_POINT` is
\[L_o=f_a E\cos\theta_i\frac{\cos\theta}{d^2}\].
Consider `min_importance = 0` because maximal incidence angle is \(\pi\), we could substitute \(d^2\) with \(\frac{r^2}{2}\) so the averaged outgoing radiance is \(f_a \frac{E}{r^2}\).
This only holds for non-transmissive surface, but should be fine to use in volume.
# EEVEE
When shading point lies outside the sphere, the sphere light is equivalent to a disk light spanning the same solid angle. The sine of the new half-angle is the tangent of the previous half-angle.
When shading point lies inside the sphere, integrating over the cosine-weighted hemisphere gives 1.0.
## Comparison with Cycles
The plane is diffuse, the blue sphere has specular component.
| Before | |After ||
|---|--|--|--|
|Cycles|EEVEE|Cycles|EEVEE|
|||||
Pull Request: https://projects.blender.org/blender/blender/pulls/108506
This fixes an issue where the light tree sampling algorithm would
discard light samples from groups of distance lights with an angle
greater than 0 when it shouldn't.
Pull Request: https://projects.blender.org/blender/blender/pulls/108832
The original names were `...update_position()`, but no update in
position is performed in these functions, rather, the entries in
`LightSample` are updated. Also make clear that the functions are used
by MNEE.
Area light sampling use special techniques to reduce noise with small
spread angles; the change in sampled area was not taken into
consideration when computing the pdf in MNEE.
Pull Request: https://projects.blender.org/blender/blender/pulls/107897
This will make further changes for light linking easier, where we want to
build multiple trees specialized for each light linking set.
It's also easier to understand than the stack used previously.
Pull Request: https://projects.blender.org/blender/blender/pulls/107560
The light tree itself is disabled on the AMD GPUs due to a compiler issue.
There are couple of places where this was not fully checked:
- The `light_sample` function in the kernel.
- The light threshold during synchronization
The former one is solved as easy as just adding an ifdef block.
The latter one is solved by delaying the threshold assignment for
later on.
Pull Request #105022
Cycles ignores the size of spot lights, therefore the illuminated area doesn't match the gizmo. This patch resolves this discrepancy.
| Before (Cycles) | After (Cycles) | Eevee
|{F14200605}|{F14200595}|{F14200600}|
This is done by scaling the ray direction by the size of the cone. The implementation of `spot_light_attenuation()` in `spot.h` matches `spot_attenuation()` in `lights_lib.glsl`.
**Test file**:
{F14200728}
Differential Revision: https://developer.blender.org/D17129
wi is the viewing direction, and wo is the illumination direction. Under this notation, BSDF sampling always samples from wi and outputs wo, which is consistent with most of the papers and mitsuba. This order is reversed compared with PBRT, although PBRT also traces from the camera.
When rendering in the viewport (or probably on instanced objects, but I didn't
test that), emissive objects whose scale is negative give the wrong value on the
"backfacing" input when multiple sampling is enabled.
The underlying problem was a corner case in how normal transformation is handled,
which is generally a bit messy.
From what I can tell, the pattern appears to be:
- If you first transform vertices to world space and then compute the normal from
them (as triangle light samping, MNEE and light tree do), you need to flip
whenever the transform has negative scale regardless of whether the transform
has been applied
- If you compute the normal in object space and then transform it to world space
(as the regular shader_setup_from_ray path does), you only need to flip if the
transform was already applied and was negative
- If you get the normal from a local intersection result (as bevel and SSS do),
you only need to flip if the transform was already applied and was negative
- If you get the normal from vertex normals, you don't need to do anything since
the host-side code does the flip for you (arguably it'd be more consistent to
do this in the kernel as well, but meh, not worth the potential slowdown)
So, this patch fixes the logic in the triangle emission code.
Also, turns out that the MNEE code had the same problem and was also having
problems in the viewport on negative-scale objects, this is also fixed now.
Differential Revision: https://developer.blender.org/D16952
